Production of hydrogen



Nov. 18, 1969 M. c. SZE ET AL PRODUCTION OF HYDROGEN Filed Aug. 4., 1964United States Patent 3,479,298 PRODUCTION OF HYDROGEN Morgan C. Sze,Garden City, N.Y., and Donald B. Stewart, Towaco, N.J., assignors to TheLummus Company, New York, N.Y., a corporation of Delaware Filed Aug. 4,1964, Ser. No. 387,292 Int. Cl. C01]: N16

US. ,Cl. 252373 8 Claims ABSTRACT OF THE DISCLOSURE Process forproducing a hydrogen-containing gas from a light hydrocarbon, such asnatural gas, wherein a natural gas-steam mixture (mole ratio of steam tonatural gas of 3.03.8:1) is reacted in a primary reformer, maintained ata ressure above 400 p.s.i.g. and a temperature above 1450 F to partiallyconvert the natural gas. The effiuent from the primary reformer is mixedwith a mixture of steam and a free oxygen-containing gas, preheated to atemperature of 1000-l500 F., in a secondary reformer to produce ahydrogen rich efiiuent, containing less than 0.5% natural gas.

This invention relates to the reforming of hydrocarbons, and moreparticularly to the steam reforming of a hydrocarbon to produce a gasrich in hydrogen, which is suitable for further treatment to formammonia, methanol, etc.

Hydrocarbon feeds generally utilized include methane, ethane, propane,butanes and/or pentanes. In most prior art processes reforming wasconducted at low pressures, i.e., 15 to 50 p.s.i.g., and at temperaturesof fro-m 1225 to 1450 F. Under such conditions steam was added to thefeed to provide a steam to carbon ratio of from about 2:1 to 4:1expressed as moles of steam to atoms of carbon. If the synthesis gas wasto be used for the production of ammonia, air is introduced at somepoint in the process to provide a gas, which, after various treatments,contain approximately three moles of hydrogen for every mole ofnitrogen.

Recent developments, particularly in the metallurgical and catalyst artshave permitted the operation of catalytic steam reformer units atincreasingly higher pressures. At such higher pressures, in order toobtain the necessary methane decomposition at reasonable tubewalltemperatures, it is necessary to operate the reformers at higher steamto carbon ratio, as for instance set forth in US. Patent No. 3,081,268wherein ratios of from 4:1 to 8:1 are described. However, operating atsuch higher steam to carbon ratios increases the duty of the radiantheating section of the reformer, thus requiring more fuel to be fired.Additionally, at such higher pressures, methane decomposition, forinstance, in the primary reformer is reduced thereby requiring thesecondary reformer to perform an increasing amount of methanedecomposition.

It is a primary object of our invention to provide an improved processfor the reforming of hydrocarbons to form gaseous streams rich inhydrogen and carbon monoxide.

A further object of our invention is to provide an improved process forthe steam reforming of hydrocarbons utilizing a process having lowerfuel requirements.

Another object of our invention is to provide an improved process forthe steam reforming of hydrocarbons at high pressures at steam andcarbon ratios of from 30:10 to 3.8:10.

Still another object of our invention is to provide an improved processfor the steam reforming of hydrocarbons at high pressures whereby theduty of the radiant heating section of the primary reformer is actuallyreduced by the lower steam to carbon ratios.

3,479,298 Patented Nov. 18, 1969 ICC A still further object of ourinvention is to provide an improved process for the steam reforming ofhydrocarbons at high pressures and lower steam to carbon ratios wherebythe methane decomposition in the secondary decomposer is easilyimproved.

Yet another object of our invention is to provide an improved processfor the steam reforming of hydrocarbons whereby wider flexibility ofoperation is achieved with greater range of feedstocks.

A further object of our invention is to provide an improved process forthe steam reforming of hydrocarbons whereby deposition of carbon on thecatalyst in the primary reformer is substantially eliminated when thehydrocarbon feed contains appreciable amounts of olefins.

Further objects of our invention will become apparent from the followingdescription when taken in conjunction with the accompanying drawingillustrating a schematic flow diagram of our improved process.

In accordance with our invention, the hydrocarbon feed such as methane,ethane, propane or other light hydrocarbons are mixed with steam at apressure of from 400 p.s.i.g. or greater, and preheated in theconvection section of a primary reformer to a temperature of above about800 F. The molal ratio of steam to atoms of carbons in the hydrocarbonfeed stream is in the range of from 3.0 to 3.8. The preheated gas streamis then passed through the catalyst tubes in the reformer wherein thetubes are heated by the combustion of a fuel gas. The primary re formereflluent is withdrawn at an outlet temperature of about 1450 F. orhigher. At this pressure and temperature, the primary reformer effluentis passed to a secondary reformer.

A mixture of steam and air, also preheated in the convection section ofthe primary reformer is introduced into the secondary reformer at atemperature in the range of 1000 to 1500 F. and mixed with the primaryreformer effluent to complete the decomposition of the methane. Theprocess gas leaving the secondary reformer is a raw ammonia synthesisgas containing hydrogen and carbon monoxide at approximately three timesthe nitrogen content, and with a methane content of 0.5 mole percent orlower. In the event that the hydrocarbon feed contains some olefins,some air is added to the steam admixed with the hydrocarbon feed to theprimary reformer so that the oxygen content of the feed to the catalysttubes is less than about 1.0 mole percent. By introducing air into thesteam admixed with the hydrocarbon feed, deposition of carbon on thecatalyst is essentially eliminated thereby permitting wider flexibilityof operation with a greater range of feedstocks.

If a mixture of steam and oxygen is introduced into the secondaryreformer, then the process gas leaving the secondary reformer is a gassuitable for further treatment to yield methanol or high purityhydrogen.

Referring now to the drawing, a hydrocarbon feed, in line 10, such asmethane, ethane, propane or other lighter hydrocarbons, is admixed withsteam in line 11 and passed to a primary reformer, generally indicatedas 13 including a convection section 14 and a radiant heating section15. Solely for the following description of the flow diagram, naturalgas will be considered to be the feed to the process, however, this isin no way intended to limit the scope of the appended claims. Thenatural gas and steam at a pressure of about 400 p.s.i.g. to about 800p.s.i.g. is preheated in the coil 16 positioned within the convectionsection 14 of reformer 13 and heated to a temperature of about 950 F. toabout 1000 F. The molal ratio of steam to carbon atoms in the feed inline 12 is controlled so that such ratio is in the range of from 3.0 to3.8.

As hereinabove mentioned, in the even that the hydrocarbon feed gascontains minor quantities of olefins, air

in line 17 is passed via line 18 under the control of valve 19, andadmixed with the steam in line 11. Olefinic hydrocarbons exhibit astrong tendency to deposit carbonaceous matters on the catalystcontained in the tubes of the radiant section of the primary reformer13. By using small amounts of air or oxygen in the feed to the radiantheating section deposition of carbon in the catalyst is substantiallyeliminated.

The preheated natural gas and steam mixture is withdrawn from the coil16 and passed through line 20 to the catalyst tubes in parallel,generally indicated as 21, vertically disposed within the radiant heatsection 15 of the reformer 13. The catalyst tubes are heated by thecombustion of a fuel in line 22 which is passed to burners,schematically illustrated as 23. 'In the radiant heating section 15 ofthe reformer 13, the preheated mixture of natural gas and steam isheated to a temperature whereby the mixture is withdrawn in line 24 fromthe radiant heating section 15 at a temperature of at least 1450 F. Theupper temperature to which the gas mixture can be heated in the radiantheating section 21 is set by present metallurgical limitations. Thegaseous effiuent in line 24 is thereafter introduced into a secondaryreformer, generally indicated as 25.

Air in line 17 is admixed with steam in line 26 and passed through line27 to a coil 28 also positioned in the convection section 14 of theprimary reformer 13. The mixture of steam and air is preheated in theconvection section 15 to a temperature in the range of from 1000 to 1400F. The preheated mixture is withdrawn through line 29 and admixed withthe gaseous mixture in line 24 in the upper portion of the secondaryreformer 25. At the temperature and pressure of the gaseous streams inlines 24 and 29, further decomposition of residual quantities of methanein line 24 is effected during passage through the catalyst bed 30 of thesecondary reformer 25. A gaseous effluent is withdrawn from thesecondary reformer through line 31 and passed to subsequent processingunits (not shown) for further treatment. The gaseous effluent in line 31contains hydrogen and carbon monoxide at approximately three times themolar content of nitrogen in such gas stream. The gaseous stream in line31 also contains less than about 0.5% methane.

By operating the primary reformer at pressures of above 400 p.s.i.g.;and at a steam to carbon ratio of from 3.0 to 3.8; and by using an airand steam mixture preheated in the primary reformer for introductioninto the secondary reformer, the duty of the radiant heating section 15of the primary reformer 13 is considerably reduced. As hereinbeforementioned, at increasing pressures, methane decomposition is reduced inthe primary reformer. Consequently, it is necessary to provide for ahigher enthalpy of the feed streams to the secondary reformer to providefor the same overall methane decomposition as obtained in the reformingprocesses heretofore operated at lower pressures.

In known prior processes, all of the steam requirements were admixedwith the hydrocarbon feed prior to passage through the catalyst tubes ofthe radiant heating section. With such processes, the total steampreheat duty was limited by the cracking temperature of the heaviesthydrocarbon constituent in the gas feed prior to contact of thehydrocarbon feed with the catalyst in the tubes of the radiant heatingsection. By preheating an air and steam mixture in the convectionsection of the primary reformer, the steam can be preheated to a muchhigher temperature. A more eflicient use is made of the fuel fired inthe radiant section of the primary reformer.

Further advantages of our process may be had by reference to thefollowing examples illustrating inter alia, the net fuel savingsobtained by reforming a hydrocarbon in accordance with our process. Inall of the following examples, 100 moles of natural gas (100% CH isintroduce-d into the process. The temperature of the feed to the radiantheating section is set at 950 F. while the outlet temperature from thesecondary reformer is maintained at 1700 F. The quantity of air which ispassed through line 17 and admixed with the steam in line 26 to thesubsequently introduced into the secondary reformer is maintained as toprovide a ratio of hydrogen and carbon monoxide to nitrogen in thegaseous effiuent withdrawn from the secondary reformer of about 3.0.

EXAMPLE I Case No 1 2 3 Pressure (p.s.i.g.) 400 400 400 Steam to primaryreformer (moles) 630 370 370 Steam to secondary reformer (moles) 0 262262 Total steam added (moles) 630 632 632 Air to secondary reformer(moles) 142. 6 142. 6 142. 6 Temp. of air+steam to secondary F.) 1,0001,000 1, 200 Gas at outlet secondary reformer:

0. S3 0. 83 0. 83 43. 9 43. 9 43. 9 292. 8 292. 9 292. 9 55. 2 55. 3 55.3 111. 3 111. 3 111. 3 H O (moles) 536.9 537.9 537.9 Fired process dutyin primary reformer (MM Btu/hr.) (radiant section) 9. 8. 580 8. 355 Netfuel saving, percent 6. 5 9. 0

There was a net fuel saving of 9% with the operation according to case 3as compared to the operation of case 1 where all of the steam is passedtogether with the feed through the primary reformer prior tointroduction to the secondary reformer.

EXAMPLE 11 Case No 1 2 3 Pressure (p.s.i.g.) 400 4.00 400 Steam toprimary reformer (moles) 1,090 370 370 Steam to secondary reformer(moles) 0 722 722 Total steam added (moles) 1,092 1, 092 Air tosecondary reformer (moles) 143. 5 143. 5 Temp. of air+steam to secondaryF. 1,000 1, 000 1, 200 Gas at outlet secondary reformer:

0H4 (moles) 0. 18 0. 18 0.18 00 (moles) 31.3 31.3 31. 3 H2 (moles) 307.7 307. 7 307. 7 C02 (moles) 68.6 68.6 68. 6 N2 (moles) 112.0 112.0 112.0H1O (molest 983.4 983.9 983.9 Fired process duty in primary reformer (MMBtu/hr.) (radiant section) 12. 398 10.712 10. 493 Net fuel saving,percent 13. 6 15. 4

The above example illustrates the additional net fuel savings whenintroducing 60% more steam to the process as compared with Example I.

EXAMPLE III Case No.. 1 2 3 Pressure (p.s.i.g.) 600 600 600 Steam toprimary reformer (moles) 630 370 370 Steam to secondary reformer (moles)0 260 260 Total steam added (moles) 630 630 630 Air to secondaryreformer (moles) 141. 3 141. 3 141. 3 Temp. 0f air+steam to secondary tF.) 1,000 1,000 1, 200 Gas at outlet secondary reformer:

CH4 (moles) 1. 77 1. 78 1. 78 00 (moles) 43. 3 43. 3 43. 3 H2 (moles).290. 3 290. 2 290. 2 C 02 (moles). 55. 0 54. 9 54.9 N2 (moles) 110.2110. 2 110.2 H2O (moles) 537. 6 536. 3 536. 3 Fired process duty inprimary reformer (MM B.t.u./hr.) (Radiant section) 9.144 8. 528 8. 310 Net fuel saving, percent- 6. 7 9. 1

Example III illustrates improved net fuel savings when operating athigher pressures but at the same quantity of steam as set forth inExample I.

As hereinbefore set forth, should it be desirable to produce a gassuitable for the synthesis of methanol, oxygen instead of air is to beadmixed with the steam in line 26. Consequently, the term free-oxygencontaining gas as used in the claims is to be interpreted as air,oxygen, or oxygen in admixture with inert gases.

It will be understood that the embodiments of the invention set forthabove are illustrative only and that certain changes may be made bythose skilled in the art within the scope of the invention as defined inthe claims appended hereto.

We claim:

1. A process for the steam reforming of light hydrocarbons whichcomprises:

(a) admixing said hydrocarbon feed and steam at a pressure above about400 p.s.i.g. to provide a feed stream having a molal ratio of steam tocarbon atoms of from 3.0:1 to 3.821;

(b) heating said feed stream to a temperature above about 1450 F. in thepresence of a reforming catalyst in a primary reforming zone to form agaseous effiuent including hydrogen;

(c) preheating a mixture of steam and a free oxyencontaining gas to atemperature of about 1000 to about 1500' F.;

(d) mixing the gaseous effluent of step (b) without further heating,with an amount of the mixture o step (c) to provide an efiluent in step(f) containing no greater than about 0.5% of light hydrocarbons;

(e) passing the resulting mixture of step ((1) through a secondaryreforming zone; and

(f) withdrawing a gaseous efiluent from said secondary reforming zonewhich is rich in hydrogen and contains no greater than about 0.5% oflight hydrocarbons.

2. The process defined in claim 1 wherein air is admixed with the steamof step (a) to provide an oxygen content of the stream passing throughthe catalyst of less than 1.0 mole percent.

3. The process defined in claim 1 wherein the free oxygen-containing gasemployed in step (c) is air and the air is present in the mixture in anamount to provide an effluent in step (f) containing about three molesof hydrogen and carbon monoxide per mole of nitrogen.

4. A process for reforming a light hydrocarbon which comprises:

(a) forming a feed mixture of said light hydrocarbon and steam at apressure above about 400 p.s.i.g. and having a molal ratio of steam tocarbon atoms of from 3.011 to 3.821;

(b) preheating said feed mixture in the convection zone of a primaryreformer to a temperature above about 800 F.;

(c) heating said preheated feed mixture in a radiant heating zone ofsaid primary reformer and in the presence of a reforming catalyst;

(d) withdrawing a gaseous effluent including hydrogen from said radiantheating zone at a temperature above about 1450 F.

(e) preheating a mixture of a free-oxygen containing gas and steam insaid convection section of said primary reformer to a temperaturebetween about 1000 F. and about 1500 F.;

(f) mixing in a secondary reforming zone the effiuent of step (d),without further heating thereof, with an amount of the mixture of step(e) to provide a secondary reforming zone efiluent containing no greaterthan about 0.5% of light hydrocarbons; and

(g) withdrawing a hydrogen-enriched gas from said secondary reformingzone containing no greater than about 0.5% of light hydrocarbons.

' 5. The process defined in claim 4 wherein the freeoxygen containinggas is air.

References Cited UNITED STATES PATENTS 1,957,743 5/1934 Wietzel et al48-l97 2,700,598 l/1955 Odell 48l96 3,264,066 8/1966 Iuartulli et a148197 X 3,278,452 10/1966 Vorum 48197 X JOSEPH SCOVRONEK, PrimaryExaminer US. Cl. X.R.

